West Nile virus (WNV) is a flavivirus and NIAID Category B infectious agent that is neurotropic and causes severe encephalitis, encephalopathy, and paralysis in humans and other animals. WNV has recently emerged into the Western hemisphere and now presents a serious worldwide public health threat. WNV and other members of the flavivirus family are arboviruses and present a biothreat through their ability to disseminate, via insect vector, throughout human population centers. Importantly, pathogenic WNV shares features of host immune defense evasion with other pathogenic members of the flavivirus family, therefore serving as a platform of study to define the virus-host interactions that control WNV/flavivirus infection. Our preliminary studies have revealed that cell tropism of WNV infection, and infection outcome, are in large part controlled by the host innate immune response and the actions of type 1 interferon (IFN) that are induced during infection. We have found that WNV and other flaviviruses trigger innate antiviral immune defenses through specific pathways of pathogen recognition within infected tissues and cells, and that the resulting IFN response triggered by these pathways differs among cells to restrict the tropism of infection. Our studies show that pathogenic WNV evades innate IFN defenses by disrupting IFN signaling, suggesting that ability to suppress the host IFN response forms the basis of neurovirulence and pathogenesis among strains. Our observations define the innate host response and virus countermeasures of IFN action as key determinants controlling immunity and pathogenesis of WNV infection. We hypothesize that viral triggering and control of the host response and IFN effector actions defines the cell tropism, virulence, and outcome of WNV infection. To investigate this hypothesis, we have designed collaborative studies that embrace the strengths of the Pacific Northwest Research Center of Excellence in Biodefense. As sub-project 1 of this programproject (P01) application, we will 1) Define the cell-specific pathogen recognition pathways that induce IFN and innate defenses against WNV infection, 2) Define the IFN-stimulated effector genes that control WNV infection, tropism and spread, and 3) Determine the molecular mechanisms by which pathogenic WNV antagonizes the host cell IFN response. These studies will integrate with P01 sub-projects 2 and 3 to determine how the host kinome and flavivirus inhibitor compounds interact with host innate immune programs to control WNV and flavivirus infection. Our work will deliver new therapeutic targets and therapy applications for the treatment of WNV and flavivirus infections.